This invention relates to a synchronous switched vertical deflection (SSVD) circuit in which the control switches each have one element grounded and in which the control elements are driven relative to ground.
A television SSVD circuit is described in U.S. Pat. No. 4,048,544 issued Sept. 13, 1977 in the name of P. Haferl. The SSVD arrangement produces vertical deflection by the use of controlled switches, which gate horizontal-frequency energy from the television horizontal deflection generator to a vertical deflection winding. The horizontal-frequency energy coupled to the vertical deflection winding is integrated to produce current in the vertical deflection winding. The switches are controlled so as to vary the amount of energy coupled from the horizontal deflection circuit to the vertical deflection winding, thereby establishing the desired vertical deflection current amplitude-time signal shape. The SSVD arrangement as described in the aforementioned Haferl patent includes first and second windings coupled to the horizontal deflection generator which produce horizontal frequency signals having mutually opposite polarities relative to a reference potential (ground). A switch in the form of an SCR type thyristor is coupled in series with each of the windings for pulse-width modulation control of the energy coupled from each source winding to the deflection winding. Such regenerative switches are advantageous by comparison with transistors because only a pulse is necessary to trigger conduction, whereas a transistor requires base drive for so long as it is to conduct. Because of the mutually opposite direction of conduction of the SCR's, the cathode of only one SCR can be coupled to reference potential, and the cathode of the other must be at a potential other than the reference potential. A conventional SCR is controlled between the gate and cathode terminals. Consequently, only one of the two SCRs controlling the vertical deflection current can have its gate driven by the control circuit with respect to ground. The other SCR has its gate and cathode electrodes floating at some potential other than the reference potential.
As described in the aforementioned Haferl patent, one of the SCR switches has its gate driven with respect to ground, while the other has its gate driven with respect to the cathode, with both voltages floating at the variable voltage across the deflection winding. Generally speaking, this requires that the control circuit be provided with an operating voltage exceeding the maximum voltage across the deflection winding solely for the purpose of allowing a direct coupling between the control circuit and the gate of the controlled SCR's. This high voltage is disadvantageous in that it undesirably increases the total power dissipation of the control circuit, and also in that it requires components capable of withstanding the greater voltage stress.
An arrangement eliminating the voltage stress on the control circuit of an SSVD system is shown in U.S. Pat. No. 4,096,415 issued June 20, 1978 in the name of P. Haferl. In this arrangement, a transformer is interposed between the control circuit and the gate-cathode path of one of the controlled SCR switches. The other SCR in this arrangement is directly coupled to the modulator of the control circuit and the gate is driven relative to ground. The pulse transformer is disadvantageous in that it is bulky, expensive, and may introduce unwanted phase shifts which perturb the stability of the system.
It is desirable to drive the control elements of the electronic switches of the SSVD circuit directly from the control circuit relative to ground.